1. Field of the Invention
The present invention relates to a silicon single crystal pull-up apparatus and a method of manufacturing a silicon single crystal and, more particularly, to a method of measuring and controlling a liquid surface level of silicon melt.
2. Description of Related Art
As a typical manufacturing method of a silicon single crystal for semiconductor devices, a Czochralski Method (CZ method) is known. In the CZ method, polycrystalline silicon nuggets are packed into a quartz crucible and heated by a heater for melting. Then, a seed crystal dipped into the obtained silicon melt is rotated with respect to the crucible while gradually pulling up the seed crystal to grow a silicon single crystal.
There may be a case where an impurity (dopant) such as arsenic (As) or antimony (Sb) is doped in a silicon melt for the purpose of adjusting the resistivity of the silicon single crystal in accordance with characteristics of a semiconductor device. The above dopant materials each have a low boiling point and are easily evaporated. In a typical silicon single crystal pull-up apparatus using the CZ method, a purge gas such as Ar or the like is supplied into a reduced-pressure chamber, so that As, Sb, SiO, As2O3, Sb2O3, or the like evaporated from the silicon melt is diffused and carried by the purge gas to contaminate the inside of the apparatus. Further, a thermal radiation shield provided in the chamber accelerates the flow rate of the purge gas flowing near the surface of the silicon melt, which unfavorably accelerates the evaporation of the dopant from the silicon melt.
Thus, there is considered a method of making the inside of the chamber into a high-pressure state in order to reduce the amount of evaporation of the dopant in the silicon melt. Further, there is proposed a method that makes the inside of the chamber into a high-pressure state, as well as, provides a cylindrical straightening member called “purging tube” inside the thermal radiation shield so as to straighten the purge gas introduced into the chamber by means of the purging tube (Japanese Patent Application Laid-Open (JP-A) No. 2007-112663).
A part of the dopant in the silicon melt is taken into the silicon single crystal due to segregation. Thus, the dopant concentration in the silicon melt becomes lower with the progress of a single crystal pull-up process, so that the dopant concentration in the silicon single crystal becomes lower, which makes it difficult to make constant the resistivity of the silicon single crystal in the pull-up axis direction. In order to make constant the dopant concentration, it is necessary to suppress the evaporation of the dopant in the initial stage of the pull-up process and to accelerate the evaporation of the dopant in the later stage thereof.
As a method for controlling the evaporation of the dopant, there is known a method using a purge gas introduced from the upper portion of a chamber. However, gas straightening effect produced by the thermal radiation shield is weak under the high-pressure condition as described above. Thus, in order to control the flow rate, etc., of the purge gas, precise control needs to be exercised with a distance between the thermal radiation shield and the melt liquid surface set small.
As a method of controlling the liquid surface level during the single crystal pull-up process, there are proposed various control methods, such as a method using a laser and a method using a camera (JP-A Nos. 2000-264779 and 2001-342095). For example, JP-A No. 2001-342095 discloses a method including previously calculating a conversion formula obtained by performing straight-line approximation for a relationship between the mirror image position of a reference reflector and liquid surface position, photographing the mirror image of the reference reflector reflected on the melt liquid surface by a camera in the actual measurement, calculating a correction value of the elevation speed of the crucible based on the measurement result and conversion formula, and controlling the liquid surface level based on a value obtained by adding the correction value to the crucible elevation speed.